1. Field of the Invention
The present invention relates to a mechanism for feeding sheets and sheet guiding equipment for conveying sheets in, for example, an image forming apparatus.
2. Related Art of the Invention
Heretofore, as shown in FIG. 1, an image forming apparatus is provided with a sheet feeding cassette 500 for storing sheets, a pick up roller 220 for feeding the sheet from the sheet feeding cassette 500, and a guide plate 230 for guiding the sheet having been fed by the pick up roller 220. On the upper central side of the guide plate 230, a sheet feeding roller 100 is provided; on the lower central side of the guide plate 230, a pad mounting plate 130 is provided; and a friction pad roller 800 mounted to the pad mounting plate 130 is urged by a spring 200 being urging means so as to abut on the sheet feeding roller 100.
Further downstream from the guide plate 230 are provided a pair of conveying rollers 240 and a pair of resist rollers 250 to feed the sheet to a photo-sensitive drum 210.
When the pick up roller 220 feeds a sheet from the sheet feeding cassette 500 toward the sheet feeding roller 100, the friction pad roller 800 abutting on the sheet feeding roller 100 moves downward by a thickness of the sheet against an urging force of the spring 200. The sheet is held between the sheet feeding roller 100 and the friction pad roller 800, and the sheet feeding roller 100 rotates in the arrow direction, thereby conveying the sheet to the downstream side. At this point, when the pick up roller 220 feeds a plurality of sheets toward the sheet feeding roller 100, though those sheets are held between the sheet feeding roller 100 and the friction pad roller 800, only the upper-most sheet is conveyed to the downstream side, so that the friction pad roller 800 prevents the multiple-feeding of the lower sheets by the friction force thereof.
However, in such a sheet feeding mechanism, when a plurality of sheets are fed as described above, the remaining sheets not conveyed are left held between the sheet feeding roller 100 and the friction pad roller 800. At this point, when there are no sheets in the sheet feeding cassette 500, the attempt to draw the sheets in order to supply sheets causes one end of the sheets to become in a fixed condition, thereby developing a first problem that the sheets are wrinkled or torn.
Further the friction pad roller 800 in such sheet feeding mechanism cannot prevent the multiple feeding if the part thereof abutting on the sheet feeding roller 100 moves in the sheet feeding direction, so that the friction pad roller 800 must be fixed to a bracket 130 so as not to be rotated, or should having a special mechanism for preventing the multiple feeding where it can be rotated, or operated in a manner to be rotated and fixed each time it feeds the sheet.
Thus, a second problem develops that where the friction pad roller 800 is fixed to the bracket 130, the part abutting on the sheet feeding roller 100 wears rapidly and the friction pad roller 800 becomes short in life, while where it can be rotated, the equipment becomes complex or its operation becomes complicated.
Further in the method of mounting the friction pad roller 800 in the sheet feeding mechanism as arranged above, when the friction pad roller 800 wears at the surface thereof and is replaced, it is necessary to remove the spring 200, replace the friction pad roller 800, and then remount the spring 200.
Accordingly, before and after the replacement of the friction pad roller 800, a difference is likely to occur in the urging force of the spring 200, thereby developing a third problem that a change in the urging force causes sheet feeding trouble such as non-sheet feeding or multiple feeding.
On the other hand, a copying machine of the prior art having a sheet feeding mechanism by which a plurality of sheet feeding cassettes are generally provided with a first conveying passage for feeding a sheet from one sheet feeding cassette thereof and a second conveying passage for feeding a sheet from the other sheet feeding cassettes, and further a third conveying passage which extends from the point where the first and the second conveying passages join toward a pair of conveying rollers to convey the sheet conveyed from the first conveying passage and that from the second conveying passage to the pair of conveying rollers provided upstream.
Now, where a sheet of the sheet feeding cassette on the first conveying passage side is selected, the sheet is conveyed by a sheet feeding roller for that sheet feeding cassette through the first conveying passage to the third conveying passage and fed to the pair of conveying rollers, while where a sheet of the sheet feeding cassette on the second conveying passage side is selected, the sheet is conveyed by a sheet feeding roller for that sheet feeding cassette through the second conveying passage to the third conveying passage and fed to the pair of conveying rollers.
However, in such sheet guiding equipment, depending on the conveying directions and the joining condition of the first, the second and the third conveying passages, when the sheet is conveyed by a first sheet feeding roller through the first conveying passage to a third conveying passage, the sheet feeding direction may change at a sharp angle, or the leading edge of the sheet conveyed through the second conveying passage may hit an inner wall of the third conveying passage to force its direction to be changed, in which case a fourth problem develops that the sheet bends and jams in the third conveying passage without being fed smoothly to the pair of conveying rollers.
On the other hand, in a sheet feeding mechanism of the prior art, as shown in FIGS. 2(a) and 2(b), where the sheet feeding roller 100 of the copying machine is replaced due to wear and the like, in order to perform efficiently the replacement, a sheet feeding shaft 300 being a rotating shaft of the sheet feeding roller 100 is pivotably supported at the central part thereof by a vertically folded end 350 of a supporting member 340, and at the right end thereof on the drawing by a body frame 390. The left end of the sheet feeding shaft 300 is free, from which the sheet feeding roller 100 is easily detachable. In order that the sheet feeding roller 100 is smoothly rotated to allow sheet feeding to be certainly performed, the supporting member 340 is joined at the end 360 vertically folded on the body frame side 390 to the body frame side 390, and at a central plate 330 to a guide plate 320. The guide plate 320 is omitted in FIG. 2(b).
Then where a sheet 310 is fed from a sheet feeding cassette 370 having a separating jaw 380 as shown in FIG. 2(a), a load applied to the sheet feeding roller 100 is small at the time of sheet feeding, whereby an effect on the guide plate 320 and the like is almost out of the question even with the supporting method as described above.
However, with the method of feeding sheets utilizing a friction pad, the load at the time of sheet feeding becomes large, whereby an abnormal sound becomes liable to occur by the sheet feeding roller 100 and the friction pad, or by the vibration of the guide plate 320 in itself. And although the abnormal sound occurring between the sheet feeding roller 100 and the friction pad can be eliminated by considering the material and shape thereof, a fifth problem develops that the abnormal sound due to the vibration of the guide plate 320 in itself cannot be eliminated.